Offshore self-elevating platforms have been widely employed in offshore exploration and production for oil or gas, as well as in other offshore markets such as for offshore electrical stations, construction support and accommodations. The self-elevating platforms usually comprise a deck structure with a top platform for providing a working area and accommodating various working instruments, and a plurality of legs along which the deck structure can be jacked up or down.
Self-elevating platforms are preferred when offshore platforms need to be installed independently on site or to be moved to new locations from time to time. Self-elevating platforms may be used in many offshore industries such as oil and gas drilling or production, construction, accommodation and so on. Several different approaches may be used for providing the elevation means required for a self-elevating platform. Hydraulic jacking systems are commonly used in self-elevating platforms for lowering the legs from an afloat condition and for raising the hull out of the water. The hydraulic jacks can be arranged in many ways to achieve this. One such arrangement is illustrated in FIGS. 1 and 2.
FIG. 1 shows a basic arrangement of a self-elevating platform. In this arrangement, the self-elevating platform 1 comprises a deck structure 10, a plurality of legs 20, and a plurality of footings 30, where the plurality of legs 20 pass through the deck structure 10 and are supported by the plurality of footings 30. In order to provide the elevating means for raising and lowering the deck structure 10 relative to the legs 20, the self-elevating platform 1 is further equipped with a jacking system 40. When installed on site, the footings 30 provide support in a seabed S and the deck structure 10 is elevated above the water level W. The plurality of legs 20 have lifting points 21 to provide engagement means between the legs 20 and the jacking system 40. In this illustration the lifting points 21 are illustrated as holes through the leg, but these could also be provided by other means known in the industry. The platform may have three or more legs, and arrangements with three, four and six legs are common.
FIG. 2 shows a schematic view of the jacking system 40 of FIG. 1. The jacking system 40 is shown using a tubular leg 20 with pin holes 21 to engage the leg. The jacking system 40 comprises a jackcase structure 41 with a first locking pin 42, a jacking yoke 43 with a second locking pin 44, and a plurality of jacking cylinders 45 with a first end and second end, where the first end of the jacking cylinders 45 is supported by the jackcase structure 41, and the second end of the jacking cylinders 45 by the jacking yoke 43. The jacking system 40 operates in a push mechanism when raising the deck structure 10 out of the water and in a pull mechanism when retrieving the legs 20 to move to a new location. The first locking pin 42 holds the legs 20 when retracting the jacking cylinder, and the second locking pin 44 holds the legs 20 during jacking. In the position shown, the second locking pin 44 is engaged while the first locking pin 42 is disengaged. In this condition the weight of the deck structure 10 is carried by the hydraulic cylinders 45.
In FIG. 2, only one locking pin is visible at each elevation (at jackcase and at yoke) for simplicity of illustration. However, it is understood that several locking pins may be employed. Common systems will usually consist of two to four locking pins at each elevation, and in some cases a larger number of pins may be used Likewise, the number of jacking cylinders can vary. In most instances, either one or two cylinders are employed for each locking pin position used.
The jacking system 40 shown in FIG. 2 allows the deck structure 10 to be lifted as described below. In this description, the procedure is described using a single leg as shown in FIG. 2. The jacking systems 40 at multiple legs would be coordinated when lifting to ensure even elevation of the deck structure 10. Where differential leg positions occur, jacking would pause at each instance that any leg reaches a retraction position, so that the cylinders on the relevant leg can be retracted and the second locking pin is re-engaged before continuing with jacking on all legs.
Beginning with the arrangement shown in FIG. 2, the second locking pin 44 is engaged and the weight of the deck structure 10 is supported by the hydraulic cylinders 45. From this position, the hydraulic cylinders 45 would be extended until a lifting point 21 is aligned with the first locking pin 42. The first locking pin 42 can then be engaged and the load transferred to the first locking pin 42 such that the second locking pin 44 can be disengaged. With the second locking pin 44 disengaged, the hydraulic cylinders then retract the yoke 43 until the second locking pin 44 is aligned with a next lifting point 21. The second locking pin 44 is then engaged and the load transferred back to the hydraulic cylinders 45 before disengaging the first locking pin 42. The hydraulic cylinders 45 can then be extended to continue with the deck raising operation. This process is repeated until the desired deck elevation is reached.
Once the deck structure 10 has been elevated, there are two options to lock the deck structure 10 in position. If the seabed S is even and footings 30 have undergone equal penetration into the seabed S, the lifting points on each leg will be aligned, and the hull can be jacked up or down until a lifting point 21 is aligned with the first locking pin 42. The first locking pin 42 can then be used to lock the platform in place and the load in the hydraulic cylinders 45 can be removed. In many cases, however, the seabed S is uneven and/or footings 30 of the platform will undergo different amounts of penetration into the seabed S. In this case, the lifting points 21 on the legs 20 across multiple legs will not align at a single elevation and so it is not possible to use the fixed pins 42 to support the deck structure 10. In this case, the hydraulic cylinders 45 must continue to support the weight of the deck structure 10 while in place.
The second scenario described above may be acceptable for short term applications; however, in longer term applications, it is desirable to provide a more direct means of securing the leg-deck connection such that the hydraulic cylinders can be stored, and/or maintained, without needing to carry the weight of the deck structure 10.